The present invention relates to a thin film field-effect transistor having an amorphous semiconductor and to a process for producing the same. A thin film field-effect transistor is hereinafter simply referred to as a thin film transistor.
A thin film transistor comprises a substrate which is made of an appropriate material, such as glass. A gate insulating film, an amorphous semiconductor layer such as an amorphous silicon layer, and source and drain electrodes are deposited on the substrate. The thin film transistor has attracted attention as a driving element for large-sized liquid crystal display devices in which liquid crystals are arranged in a matrix form.
The prior art is explained with reference to FIGS. 1 and 2.
In FIG. 1, the source electrodes S and the gate films G delineate the lateral and vertical lines of the matrix. The drain electrodes D are rectangular and have a large cross section. An opposite electrode ITO (FIG. 2) is arranged at the side of a liquid-crystal panel opposite to the drain electrodes D. As is shown in FIG. 2, the drain electrode D and the opposite electrode ITO form a pair of electrodes of the liquid-crystal panel. Glass plates GS are disposed at the outermost side of the liquid-crystal panel, and a liquid crystal L is sealed between the drain electrode D and the opposite electrode ITO. The distance between the drain electrode D and the opposite electrode ITO is approximately 10 .mu.m.
The operation of the liquid-crystal display device shown in FIGS. 1 and 2 is now explained. When a voltage is applied to a predetermined source electrode S and to a predetermined gate electrode G, the predetermined source electrode S, gate electrode G, and drain electrode D, which form a thin film transistor, are energized, with the result that a part of the liquid crystal L becomes transparent because it is rearranged between the opposite electrode ITO and the predetermined drain electrode D. In order to obtain a fine image, a number of image-forming elements is necessary. In addition, in order to obtain a picture plane of a certain dimension, for example, size A-4, a number of driving units is necessary. Thin film transistors are appropriate driving units for obtaining an A-4 size picture plane having a fine image. However, integrated circuit chips a few millimeters in size are inappropriate driving units.
A known thin film transistor is explained with reference to FIG. 3, in which reference numeral 1 denotes a glass substrate, 2 a gate electrode made of metal, such as NiCr, 3 a gate insulating film made of SiO.sub.2 or the like, 4 an amorphous silicon layer, 5 a source electrode, and 6 a drain electrode. The source electrode 5 and the drain electrode 6 are electrically conductive through a channel (not shown) which is formed in the amorphous silicon layer 4 when a voltage is applied to the gate electrode 2. The channel is formed above the gate electrode 2.
The known thin film transistor is produced by a procedure in which metal is deposited on the glass substrate 1 and is then delineated in the form of a gate electrode 2. Next, SiO.sub.2 is grown by means of chemical vapor deposition (CVD) and is then delineated in the form of a gate insulating film 3. Subsequently, amorphous silicon is deposited on the entire top surface of the glass substrate 1 and is then delineated in the form of an amorphous silicon layer 4. Finally, the material of the source electrode 5 and the drain electrode 6 is deposited on the entire top surface of the glass substrate 1.
In the procedure described above, it is difficult to precisely align the gate, source, and drain electrodes. If the ends of the source electrode 5 and the drain electrode 6 are positioned outside the ends of the gate electrode 2, the source electrode 5 and the drain electrode 6 cannot be electrically connected to each other. Therefore, the thin film transistor is kept turned off even when a voltage is applied to the gate electrode 2. If the end of the source electrode 5 or the drain electrode 6 is positioned centrally, that is, if one of these electrodes overlaps the gate electrode 2, the coupling capacitance between the gate electrode 2 and the source electrode 5 or drain electrode 6 is increased so that the responding speed of the thin film transistor becomes slow.